A number of receivers detect signals by comparing the signals which are received with a predetermined power level. Based on this comparison, only those signals which have a power level greater than a predetermined threshold level are detected and processed. For example, threshold-detection receivers of conventional radar and laser radar (ladar) systems detect return signals which were transmitted by a radar transmitter, which have reflected from a distant object and which have a power level greater than a predetermined threshold level. In addition, certain types of communications systems employ threshold-detection receivers to detect communications signals which were transmitted by a communications source, such as a radio transmitter, and which have a power level greater than a predetermined threshold level.
In addition to the true signals, such as the return or reflected signals in a radar or ladar system and the communications signals in a communications system, there are numerous other signal sources which emit extraneous signals which a receiver can detect. For example, the frequency bandwidths in which radar, ladar and communications systems operate typically include numerous sources of noise. The noise sources can produce a variety of noise signals, at least some of which have a power level which exceeds the predetermined threshold level so as to be detected and processed by a threshold-detection receiver. Although the detected noise signals have a power level greater than the predetermined threshold level, the noise signals do not provide additional, useful information. Instead, the noise signals may distort or otherwise dilute the reception of true signals by the receiver.
Accordingly, designers and operators of systems which employ threshold-detection receivers in relatively noisy environments must perform a delicate balancing act, That is, the predetermined threshold level must be low enough such that a relatively high percentage, if not all, of the true signals received by the receiver have a power level greater than the predetermined threshold level and are detected and processed by the receiver. It will be apparent to those skilled in the art, however, that the lower the predetermined threshold level is set, the more noise signals which have a power level exceeding the predetermined threshold level will be detected by the receiver, thereby further skewing the results.
Accordingly, designers and operators of systems which employ threshold-detection receivers typically design such systems to operate effectively at a predetermined false alarm rate. As known to those skilled in the art, the false alarm rate is the rate at which noise or other extraneous signals are detected by a threshold-detection receiver. Preferably, the predetermined false alarm rate remains constant to further improve the detection of the true signals.
Typically, the false alarm rate is based upon the noise statistics, the ratio of the predetermined threshold level to the root mean square of the noise voltage and the bandwidth of the receiver. Since noise statistics can vary dramatically between noise sources, the noise statistics generally fluctuate as different noise sources emerge. In addition, the noise statistics can change significantly with variations in the temperature, time, background, jamming and other variables. Consequently, it has been relatively difficult to obtain the constant false alarm rate in systems employing threshold-detection receivers due to, among other things, the sizable fluctuations in the noise statistics.
Nonetheless, several methods have been proposed which attempt to maintain constant false alarm rates for threshold-detection receivers. According to one conventional approach, the predetermined threshold level is based upon a multiple of the average power of the received signals. However, signals which have very high power levels may skew the predetermined threshold level such that the threshold level is set to an excessively high level, thereby potentially causing the receiver to miss or fail to detect several true returns. In addition, by not measuring and controlling the true false alarm rate, but, instead, measuring a parameter related to the false alarm rate, i.e., the average power of the received signals, the true false alarm rate of this approach can vary greatly, such as by a factor of 100, as the percentage of true signals detected by the threshold-detection receiver varies between 0% and 100%.
Further, there are a variety of types of noise signals. For example, a first type of noise signal can include those noise signals introduced by the various components of the receiver, including the filter and the square log detector. In addition, a second type of noise signals is Gaussian noise. These different noise types are generally independent such that both types of noise signal can vary irrespective of the other type of noise signal.
Thus, the conventional approach of basing the predetermined threshold level on a multiple of the average power of the received signals does not take into account the different types of noise signals and the various independent fluctuations which each of the types of noise signals can undergo. Instead, this approach is typically based upon the assumption that the noise statistics are independent of the noise source. In addition, this approach generally assumes that the noise statistics do not change with time such that an average power of the received signals can be determined over one time period and can thereafter be employed in subsequent time periods to determine the predetermined threshold level.
A second method of obtaining a constant false alarm rate for a threshold-detection receiver is generally employed by systems having a relatively low pulse rate and a limited range, such as a ladar system operating at 10 Hz and having a maximum range of 10 kilometers. Due to the relatively low pulse rate and limited range, there is a relatively long dead time between the reception of a true signal and the transmission of the succeeding signal. For example, in the exemplary ladar system operating at 10 Hz and having a maximum range of 10 kilometers, each true signal, i.e., each return or reflected signal, would be received within much less than 1 millisecond. However, the succeeding signal would not be transmitted for at least another 99 milliseconds.
According to this second method of obtaining a constant false alarm rate, all of the signals received by the threshold-detection receiver within the dead period between pulses represents undesirable noise signals. Based upon the noise signals detected in the dead period, the true false alarm rate can be measured. However, this method cannot effectively be employed with threshold-detection receivers which operate at relatively high sampling rates, that is, threshold-detection receivers which could receive a true signal at virtually any time.